The influence of image magnification in first-trimester screening for Down syndrome by fetal nuchal translucency in Asians

Simplified protocol of nuchal translucency measurement: Is it still effective?

Objective

Nuchal translucency (NT) is the most powerful screening tool for Down syndrome and congenital cardiac anomaly, therefore strict guidelines were established to get accurate NT values. However, to stick to the guideline in all pregnant women is time-consuming and superfluous in majority of low risk population. We undertook this study to investigate whether the simplified protocol enables to select low risk group and is effective in them even if we skip the suggested NT measurement.

Methods

NT and crown-rump length (CRL) were measured prospectively. First, CRL was measured in the ordinary view that was mid-sagittal section of fetus in neutral position, and NT was measured at the same frozen screen (first measured value, 1MV). Then, NT was measured again according to the Fetal Medicine Foundation (FMF) guideline (second measured value, 2MV).

Results

There was good correlation between 1MV and 2MV in each case (r = 0.83, P < 0.001). All of the NT values over the 95th percentile in 2MV also belonged to over the 95th percentile in 1MV. NT value of 2 mm in 1MV could be used as a cut-off to obtain over the 95th percentile 2MV by receiver operating characteristic curve (sensitivity 100%, specificity 80.5%). The proportion of 1MV ≥ 2 mm was only 23.8% of all cases, namely we had only to measure 2MV in 23.8% patients. Every 95th percentile or more 2MV could be detected with this simplified protocol.

Conclusion

If NT is less than 2 mm at ordinary CRL view, we may skip suggested NT measurement according to FMF guideline.

Semi-automated system for measurement of nuchal translucency thickness

OBJECTIVE

To estimate intersonographer and intrasonographer variance components of fetal nuchal translucency (NT) thickness measurement using the traditional manual approach and a new semi-automated system.

METHODS

A semi-automated method was developed for measurement of the NT. In this method, the operator places an adjustable box over the relevant area at the back of the fetal neck. The system draws a line through the center of the nuchal membrane and another line at the edge of the soft tissue overlying the cervical spine. The system then identifies the largest vertical distance between the two lines. The images of 12 fetuses at 11-13 weeks of gestation satisfying the guidelines of The Fetal Medicine Foundation for measurement of NT were selected. They were exported in DICOM format from the ultrasound system, and four versions of each image were stored under different names. The resulting 48 images were presented in random order for electronic assessment. A total of 20 sonographers measured the NT in each set of 48 pictures, twice using the semi-automated system and twice using the manual system, according to a randomized block design. Within- and between-operator variance components were estimated. Relative biases were assessed by comparing the means from the two methods.

RESULTS

The estimated between-operator SD using the semi-automated method was 0.0149 mm compared with 0.109 mm for the manual method. The respective within-operator SD values were 0.05 mm and 0.126 mm. The intraclass correlation coefficients for different sonographers measuring the same images were 0.98 and 0.85 for the semi-automated method and the manual method, respectively.

CONCLUSION

The measurement of fetal NT is more reliable when a semi-automatic approach is used rather than the traditional manual method.

Variability of Nuchal Translucency Measurement

OBJECTIVES

To evaluate intra- and inter-observer variations of nuchal translucency (NT) measurements and study the duration of measurements.

SUBJECTS

One hundred and forty-seven singleton pregnant women with 10-14 weeks of gestation who had attended antenatal clinic during January 1st, 2000-August 31st, 2001 were included.

METHODS

Crown-rump length and NT were measured three times for each woman. Then, another examiner who was unaware of the previous results did the other three measurements. The duration of measurements was recorded. Intra-observer and inter-observer variations were analyzed by repeated ANOVA and paired t test, respectively. The correlation of NT measurements within each observer and between paired observers was evaluated by intraclass correlation coefficients (ICC). Kappa statistic was calculated for agreement.

RESULTS

Intra-observer variation of all 10 examiners at three institutes was varied from 0.20 +/- 0.27 to 0.33 +/- 0.41 mm. Inter-observer variation at Chiang Mai University was 0.40 +/- 0.37 mm which was the highest value compared with those at Prince of Songkla and Khon Kaen University (p < 0.05). The mean duration of measurements was in range of 8-12 min. The intra-observer repeatability of the first two measurements showed ICCs varying from 0.61 to 0.94. The inter-observer repeatability of paired examiners showed a variation in ICC from 0.28-0.90. The kappa value expressing the intra- and inter-observer repeatability as being >95th or < or = 95th percentile was 0.73 and 0.72, respectively.

CONCLUSIONS

NT measurement was reproducible for small variation and good agreement. The duration of measurements was acceptable.